The present invention related generally to the field of materials technology, and more specifically to ceramic matrix composite materials, and in particular to a ceramic matrix composite material having improved interlaminar strength and a method of manufacturing such material.
Ceramic materials generally have excellent hardness, heat resistance, abrasion resistance, and corrosion resistance, and are therefore desirable for high temperature machine applications such as gas turbines and the like. However, ceramic materials are easily fractured by tensile stresses and exhibit a high degree of brittleness. To improve upon the fracture toughness of a ceramic material, it is known to provide a ceramic matrix composite (CMC) material wherein a plurality of inorganic or metal fibers are disposed in a matrix of ceramic material. The fibers provide tensile strength and toughness to augment the other desirable properties of the ceramic material. A CMC material may be formed by impregnating a preform of fiber-containing fabric material with ceramic material powder using a known wet method such as slip casting or slurry infiltration. The cast or laid-up part is then dried using low pressures and temperatures to form a green body. The green body is then sintered by known techniques such as atmospheric-pressure sintering or reaction sintering to sinter the matrix to its final density to form the ceramic matrix composite material. One example of a commercially available oxide fiber/oxide matrix CMC material is a Nextel 720 fiber/alumina matrix composite available from COI Ceramics, Inc. of San Diego, Calif.
One of the limitations in the application of ceramic matrix composite materials to combustion turbine applications is the available interlaminar shear and tensile strength of the composite. For many such applications, the predicted interlaminar stresses exceed the design allowable limits for commercially available materials. Methods for improving these properties are therefore needed.
One possibility for improving the interlaminar tensile strength of a CMC material is matrix densification. Current oxide CMC""s are made using a one-step matrix processing which yields a high level of porosity. This porosity gives the composite maximum in-plane strength, strain tolerance and notch insensitivity. Increasing matrix density by additional infiltration steps would improve the matrix-dominated properties (interlaminar shear and tension) of the composite. However, increased matrix density has been shown to dramatically decrease the desirable in-plane properties and would result in a more brittle failure mode for the material.
Another way to improve the interlaminar tensile strength of a CMC material is to incorporate a fiber coating. For non-oxide CMC""s, a weak interface coating on the fiber has been shown to improve load distribution from the matrix to the fibers and to yield a tough, high strength composite. Work is in progress to develop weak fiber/matrix interface coatings for oxide-based CMC""s. These coatings are still under development and their benefits have yet to be demonstrated for this class of material.
A third approach to improved interlaminar strength is 3D fiber reinforcement. Three dimensional woven and braided fiber preforms are commercially available and would serve to improve the interlaminar properties of the composite. In order to achieve this benefit, some compromise of in-plane fiber orientation and volume fraction is necessary, resulting in degraded in-plane properties. In spite of many years of research and demonstration, the manufacture of 3D woven preforms has yet to be automated. High preform cost due to a labor intensive process, operator-dependent quality issues, and high capital outlay required for dedicated machinery are all barriers to the commercialization of woven 3D preforms. Three dimensional braided preforms, while more automated, are size and complexity limited by the equipment that is available today. In addition, all 3D reinforcement options require the development of new matrix infiltration methods and compositions, thereby moving away from the commercial lamination method of wet prepreg lay-up.
Rather than full 3D preforming, it is known to stitch 2D laminates with ceramic fibers to obtain a 3D fiber. This method has been performed commercially, but only on dry preforms and on fairly simple geometries. The action of stitching damages both in-plane fibers and through-thickness fibers so that the advantage gained is less than predicted based on fiber volume fraction estimates. The limitation of the method to dry preforms means that new matrix infiltration processes would need to be developed to accommodate such stitching. Importantly, complex shapes needed for many applications, such as turbine vane leading edges, are beyond the capability of existing methods.
Accordingly, there is a need for improving the interlaminar shear and tensile strength of a ceramic matrix composite material without degrading the in-plane properties of the material. There is a further need for a method of improving the interlaminar shear and tensile strength of a ceramic matrix composite material that is commercially practical and that can be implemented with existing infiltration processes.
An improved material is described herein as including a composite material having a plurality of layers of fibers disposed within a matrix material, and a plurality of stitches of recast composite material disposed through a thickness of the composite material transverse to the layers of fibers. Each stitch in the improved material may be a generally tubular shaped zone of recast composite material defining a hole formed in the composite material. A filler material may be disposed within the hole, and such filler material may be an adhesive material or may be one of the group of a phosphate-based composition having oxide ceramic filler particles and a silica-based composition having oxide ceramic filler particles. If the nominal cross-sectional diameter of the stitches is defined to be d, and the nominal distance between stitch centerlines is defined to be D, then D divided by d may be in the range of 4 to 7. For high temperature applications, the material may have a layer of thermally insulating material disposed on the composite material. For such an embodiment, each stitch may be a zone of recast composite material defining a hole extending through the composite material, with the hole extending into the layer of thermally insulating material; or each stitch may be a zone of recast composite material defining a hole extending through less than the full thickness of the composite material, with the hole extending through the layer of thermally insulating material.
A method of producing a material is described herein as including: providing a ceramic composite material having a plurality of layers of ceramic fibers disposed within a ceramic matrix material; forming a plurality of stitches through a thickness of the composite material in a direction transverse to the layers, each stitch comprising a zone of recast composite material. The method may further include forming the stitches in a pattern selected to achieve a predetermined level of interlaminar strength in the material or in a pattern selected to achieve a predetermined level of thermal conductivity through the material in a direction transverse to the layers. Advantageously, the method may include: forming each of the stitches by directing laser energy into the composite material to form a volume of melted composite material; and allowing the melted composite material to cool to form the zone of recast composite material. In one embodiment, the method includes: forming each of the stitches by directing laser energy into the composite material to form a hole surrounded by the zone of recast composite material; and filling the hole with a filler material, such as a ceramic adhesive material or a unidirectional fiber reinforced prepreg material that is cured in situ within the hole. The holes may be filled with one of a phosphate-based composition having oxide ceramic filler particles and a silica-based composition having oxide ceramic filler particles. Such holes may be formed prior to or after a step of densification of the matrix material. In one embodiment, the method may include the further step of disposing a layer of thermally insulating material on the composite material, and directing laser energy into the composite material and layer of thermally insulating material to form a plurality of holes each surrounded in the composite material by a zone of recast composite material. The holes may be formed in this embodiment to extend completely through the composite material and only partially through the layer of thermally insulating material, or completely through the thermally insulating material and only partially through the composite material. The stitches may be formed before or after the step of disposing a layer of thermally insulating material on the composite material.